The present invention relates to a vibration type transducer for measuring a density or a pressure of a fluid to be measured by detecting a resonant frequency of a vibratory plate which is in contact with the fluid.
The present inventors have proposed a vibration type transducer disclosed in Japanese Patent Publication No. 239228/1985 and U.S. patent application Ser. No. 922,694 now U.S. Pat. No. 4,872,335 which measures the density or the pressure of the fluid by utilizing the relationship wherein a resonant frequency of a composite vibration system varies in accordance with an inertance at a fluid introducing port, i.e., depending on the density of the fluid to be measured. This composite vibration system is composed of a disk-like mechanical vibration part and an acoustic vibration system including a cavity contactually formed in at least one surface of the mechanical vibration part and the fluid introducing port for introducing the fluid into the cavity.
FIG. 1 is a longitudinal cross-sectional view illustrating the principal portion of a vibration type transducer associated with the above-mentioned proposal. FIG. 2 is a block diagram of the transducer depicted in FIG. 1. In FIGS. 1 and 2, a vibratory member with a cylindrical bottom, generally indicated at 1, is constructed such that a disk-like piezoelectric vibrator 2 is fixedly bonded to an inner surface of a bottom 1a, and an opening end is provided with a collar 1b. The vibratory member 1 is formed of a metal thin film which is approximately 0.1 mm in thickness. The piezoelectric vibrator 2 is constituted by a disk-like piezoelectric substrate 2a having a thickness of approximately 0.2 mm, a first electrode formed on one surface of the substrate 2a, a second electrode 2c and a third electrode 2d which are formed on the other surface of the substrate 2a. The surface of the substrate 2a on which the first electrode 2b is disposed is attached to the bottom 1a of the vibratory member 1. The electrode 2b is electrically connected to the vibratory member 1.
The reference numeral 3 designates a mechanical vibration part consisting of the cylindrical vibratory member 1 and the piezoelectric vibrator 2. The numeral 4 represents a cylindrical bottomed container including a female thread 4a formed in the opening end thereof. One end of a cylindrical tube 5 is fixed to an outer surface of a bottom 4b of this container. The bottom 4b of the container is formed with a circular through-hole 4c through which the inside of the tube 5 communicates with the inside of the container 4. The through-hole 4c has a diameter equal to the inside diameter of the tube 5.
The numeral 6 denotes a cylindrical bottom housing designed for fixing the vibratory member 1. The collar 1b is sandwiched in between the container 4 and the housing 6 by causing a male thread 6a formed on an outer surface of the housing 6 to screw into the female thread 4a, thus fixing the vibratory member 1. A bottom 6b of the housing 6 is formed with a through-hole 6c. A printed circuit substrate 8 constituting of a detecting circuit 7 is fixedly bonded to an inner surface of the bottom 6c. A first cavity generally designated at 9 is surrounded by the vibratory member 1 and the housing 6. The cavities 9 and 10 are partitioned by the vibratory member 1 so as to have a high density of fluid. The symbols 11b and 11c denote conductors through which the electrodes 2c and 2d of the piezoelectric vibrator 2 and the vibratory member 1 are connected to the detecting circuit 7.
Next, the operation of the transducer illustrated in FIG. 1 will be explained. To start with, the transducer is placed in a fluid 12 to be measured. The fluid 12 flows via the tube 5 into the cavity 9 and via the through-hole 6c into the cavity 10. Immediately when the detecting circuit 7 is charged with the electricity, an AC voltage 13a is outputted from AC amplifier 13. The voltage 13a is impressed on the second electrode 2c of the piezoelectric vibrator 2. The piezoelectric substrate 2a is arranged to expand and contract in the radial direction, depending on the positive and negative of the voltage 13a. As a result, the bottom 1a of the vibratory member 1 performs flexural vibrations in the axial direction of the tube 5 of the vibratory member. An AC voltage on line 14 is produced corresponding to distortion of the piezoelectric substrate 2a between the first electrode 2b and the third electrode 2d of the vibrator 2. This voltage on line 14 is input to a feedback circuit 15 which effects a positive feedback of an output voltage on line 15a to the AC amplifier 13.
It follows that the bottom 1a of the vibratory member performs self-oscillations while maintaining a flexural vibration state in which the resonance takes place at the natural oscillation frequency F of a composite vibration system 18 consisting of: a first acoustic vibration system 16 formed by the first cavity 9, the through-hole 4c and the internal portion of the tube 5, this vibration system 16 is based on the fluid 12 introduced into a communication space; the mechanical vibratory part 3; and a second acoustic vibration system 17 formed by the second cavity 10 and the through-hole 6c, this vibration system 17 being based on the fluid 12 introduced into the communication space. In this case, the oscillation frequency F corresponds, as will be discussed below to a density .rho. of the fluid 12 to be measured and further accords with a frequency of the AC voltage on line 13a. The density .rho. can therefore be measured by obtaining the frequency of the output voltage on line 13a. An output circuit generally indicated at 19 receives the input of the voltage 13a and is intended to facilitate the measurement of the oscillation frequency F by outputting a pulse train signal 19a having a pulse frequency identical with the frequency of the voltage 13a. The detecting circuit 7 comprises the amplifier 13, the feedback circuit 15 and the output circuit 19.
Next, the fact that the oscillation frequency F is a function of the density .rho. will be explained. The transducer depicted in FIG. 1 is, as explained earlier, defined as the composite vibration system consisting of the mechanical vibration part 3 serving as a mechanical vibration system, the first acoustic vibration system 16 and the second acoustic vibration system 17. As illustrated in FIG. 3, an electrical equivalent circuit illustrates how the vibration system 18 converts the acoustic vibration system 16 and 17 into the mechanical vibration system. In FIG. 3, the symbols Mm and Cm respectively denote an effective mass and a compliance of the vibration part 3; the symbol M.sub.a1 represents an inertance of the fluild 12 in the tube 5 C.sub.a1 stands for an acoustic capacity of the fluid 12 within the first cavity 9; M.sub.a2 designates an inertance of the fluid 12 in the through-hole 6c; and C.sub.a2 denotes an acoustic capacity of the fluid 12 within the second cavity 10. The symbols a.sub.1 and a.sub.2 indicate the conversion coefficient for converting individual constants for the acoustic vibration of the mechanical vibration system. The above-mentioned acoustic capacities C.sub.a1 and C.sub.a2, and the inertances M.sub.a1 and M.sub.a.sub.2 are expressed in the following formulae (1) and (2). EQU C.sub.a1 =V.sub.1 /(.rho.C.sup.2), M.sub.a1 =(.rho.l.sub.1)/S.sub.1 ( 1) EQU C.sub.a2 =V.sub.2 /(.rho.C.sup.2), M.sub.a2 =(.rho.l.sub.1)/S.sub.2 ( 2)
where V.sub.1 and V.sub.2 are the volumes of the cavities 9 and 10; c is the sound velocity; l.sub.1 and S.sub.1 are the length and the sectional area, respectively, of the tube 5; and l.sub.2 and S.sub.2 are the length and the sectional area, respectively, of the through-hole 6.
FIG. 3 explains the transducer illustrated in FIG. 1. The acoustic capacity C.sub.a2 is made to increase, whereas the inertance M.sub.a2 is made to decrease. The principal portion of the transducer is constructed so that a resonant frequency F.sub.a2 of the vibration system which is composed of C.sub.a2 and M.sub.a2 is considerably lower than a resonant frequency Fm of the vibration system which is composed of Cm and Mm. Hence, FIG. 3 is redrawn into FIG. 4.
FIG. 4 is redrawn into FIG. 5 where an acoustic compliance (.sub.ca1 /a.sub.1.sup.2) and an acoustic mass (a.sub.1.sup.2.M.sub.a1) are selected to establish formula (4) with respect to an angular frequency .omega..sub.n expressed by the formula (3). As is obvious from FIG. 5, .omega..sub.n is the resonant angular frequency of the circuit depicted in FIG. 5. EQU .omega..sub.n.sup.2 =[1/(Cm.(Mm+a.sub.1.sup.2.M.sub.a1))] (3) EQU 1/(.omega..sub.n.(C.sub.a1 /a.sub.1.sup.2))&gt;&gt;.omega..sub.n (a.sub.1.sup.2.M.sub.a1) (4)
The formula (5) is obtained from the formulae (3) and (4). However, for sufficient sensitivity in measurement of the density, it is usually required to establish the conditions of formula (6). Therefore, the formula (7) is deduced from the formulae (5) and (6) as follows: ##EQU1##
The principal portion of the transducer illustrated in FIG. 1 is constructed to make the formulae (6) and (7) valid. Consequently, the formula (4) is established through the formula (5). In this case, the electrical equivalent circuit of the composite vibration system is therefore expressed in FIG. 5. It follows that the vibration system 18 continues the self-oscillations at the resonant frequency F corresponding to .omega..sub.n of the formula (3). However, it is evident from the formulae (1) and (3) that .omega..sub.n is the function of the density .rho.. Therefore, in the transducer depicted in FIGS. 1 and 2, the density .rho. of the fluid 13 can be measured on the basis of the pulse frequency of the signal 19a outputted from the detecting circuit 7.
The transducer illustrated in FIG. 1 has an advantage in that the density can be measured with the above-described simple apparatus. However, the effective mass Mm of the mechanical vibration part 3 increases because the piezoelectric vibrator 2 has a disk-like configuration. As can be understood from the formula (3), this creates a problem wherein the sensitivity in measurement of the density is low.